1. Field of the Invention
The present invention relates to thin films of quasicrystalline alloys, their preparation and their uses.
2. Description of Related Art
Quasicrystalline alloys, which are alloys consisting of one or more quasicrystalline phases, are known. Quasicrystalline phase is understood to mean a quasicrystalline phase in the strict sense, or an approximant phase. A quasicrystalline phase in the strict sense is a phase exhibiting a rotational symmetry normally incompatible with the translational symmetry, i.e. a symmetry with a 5-, 8-, 10- or 12-fold rotation axis, these symmetries being revealed by the diffraction of radiation. By way of example, mention may be made of the icosahedral phase of point group m3 5 and the decagonal phase of point group 10/mmm. An approximant phase or approximant compound is a true crystal insofar as its crystallographic structure remains compatible with the translational symmetry, but it has, in the electron diffraction photograph, diffraction patterns whose symmetry is close to that of 5-, 8-, 10-, or 12-fold rotation axes. By way of example, mention may be made of the orthorhombic phase O.sub.1, characteristic of an alloy having the atomic composition Al.sub.65 Cu.sub.20 Fe.sub.10 Cr.sub.5, the unit cell parameters of which are: a.sub.o.sup.(1) =2.366, b.sub.o.sup.(1) =1.267 and c.sub.o.sup.(1) =3.252 in nanometer. This orthorhombic phase O.sub.1 is called the approximant: of the decagonal phase. Moreover, this is so closely so that it is not possible to distinguish its X-ray diffraction pattern from that of the decagonal phase. It is also possible to mention the rhombohedral phase having parameters a.sub.R =3.208 nm, .alpha.=36.degree. Al.sub.64 Cu.sub.24 Fe.sub.12 in terms of number of atoms. This phase is an approximant, phase of the icosahedral phase. It is also possible to mention orthorhombic phases O.sub.2 and O.sub.3 having respective parameters a.sub.o.sup.(2) =3.83; b.sub.o.sup.(2) =0.41; c.sub.o.sup.(2) =5.26 and a.sub.o.sup.(3) =3.25; b.sub.o.sup.(3) =0.41; c.sub.o.sup.(3) =9.8 in nanometer, which are present in an alloy of composition Al.sub.63 Cu.sub.17.5 Co.sub.17.5 Si.sub.2 in terms of number of atoms or else the orthorhombic phase O.sub.4 having parameters a.sub.o.sup.(4) =1.46; b.sub.o.sup.(4) =1.23; c.sub.o.sup.(4) =1.24 in nanometer, which is formed in the alloy of composition Al.sub.63 Cu.sub.8 Fe.sub.12 Cr.sub.12 in terms of number of atoms. It is also possible to mention a C phase of cubic structure, very often observed to coexist with the true quasicrystalline or approximant phases. This phase, which is formed in certain Al-Cu-Fe and Al-Cu-Fe-Cr alloys, consists of a superstructure, due to the effect of chemical order of the alloy elements with respect to the aluminum sites, of a phase of structure of the Cs-Cl type and having a lattice parameter a.sub.1 =0.297 nm. Mention may also be made of an H phase of hexagonal structure which derives directly Erom the C phase as demonstrated by the epitaxial relationships, observed using electron microscopy, between crystals of the C and H phases and the simple relationships which connect the crystalline lattice parameters, namely a.sub.H =3.sqroot.2a.sub.1 / .sqroot.3 (to within 4.5%) and c.sub.H =3.sqroot.3a.sub.1 /2 (to within 2.5 %). This phase is isotypic of a hexagonal phase, denoted by .PHI.AlMn, discovered in Al-Mn alloys containing 40% by weight of Mn. The cubic phase, its superstructures and the phases which derive therefrom constitute a class of approximant phases of the quasicrystalline phases of neighboring compositions. For more information about quasicrystalline phases in the strict sense, and their approximants, reference may be made to WO 93/13237 (J. M. Dubois, et al.).
Certain quasicrystalline alloys have been prepared in the form of thin films. Thin films of quasicrystalline alloys Al.sub.62.5 Cu.sub.25 Fe.sub.12.5, prepared by sputtering followed by a heat treatment, which have a thickness of 0.2 to 0.3 .mu.m and a high electrical resistivity, have been described by T. Klein et al., Appl. Phys. Letters, 64, 431, 1994. Thin films of a quasicrystalline alloy Al.sub.65 Cu.sub.20 Fe.sub.15 having a thickness of about 10 .mu.m obtained by vacuum evaporation have been described by C. L. Chien et al., Phys. Rev. B 45 12793 (1992). Thin films of alloy Al.sub.62 Cu.sub.20 Co.sub.15 Si.sub.3 having a thickness of 100-500 .ANG. obtained by vacuum evaporation have been described by J. Reyes-Gasga, Thin Solid Films, 220 (1992) 172-176. Films of quasicrystalline alloy having a thickness of 0.5 .mu.m, prepared by a laser ablation technique using a target having the composition Al.sub.70 Pd.sub.22 Mn.sub.8 or Al71Pd.sub.20 Mn.sub.9 or Al.sub.70 Pd.sub.10 Mn.sub.20 have been described by N. Ichigawa, et al., Jpn. Appl. Phys. Vol. 33, L736-738 (1994).